As a material for a food container such as a cup or a tray, a blister pack, a container for hot-fill, a plastic case, or a tray or a carrier tape for transporting electronic parts, polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyethylene terephthalate, or the like have been used. These plastic products or the like are generally discarded after use, so that disposal such as burning and burial when discarding after use poses a problem. Resins such as polyethylene, polypropylene and polystyrene are large in calorific value when burnt, so that they may damage incinerators during burning treatment, and polyvinyl chloride produces harmful gas during burning.
On the other hand, for burial disposal, too, since these plastic products are high in chemical stability, they will barely decompose in the natural environment and remain semipermanently in the earth, so that they will saturate the capacity of garbage disposal sites in a short time. Also, if discarded in the natural environment, they will mar the landscape or destroy the life environment of marine lives or the like.
Therefore, from the viewpoint of environmental protection, in recent years, studies and developments of biodegradable materials are being actively done. As one of biodegradable materials that are gathering attention, there is a polylactic acid. Since a polylactic acid-family resin is biodegradable, hydrolysis proceeds naturally in the earth and water, so that it becomes a harmless decomposed product. Also, since it is small in burning calorie, even if it is subjected to burning treatment, it will not damage furnaces.
Further, since the starting material originates from a plant, it does not-resort to oil resources which is exhausting.
But a polylactic acid-family resin is low in heat resistance, and was not suitable for use at high temperature such as containers for heated foods or containers into which hot water is to be poured. Also, in storing or transporting polylactic acid-family resin sheets or their formed bodies, the interiors of storages, transporting trucks and ships often reach high temperature in summer, so that problems of deformation, fusing, etc. occur.
As a technique for imparting heat resistance to a polylactic acid-family resin, there is a method in which in a forming step, the heat resistance is imparted by crystallizing a polylatic acid to a high degree in a mold by holding the mold near the crystallizing temperature of the polylactic acid-family resin (80-130° C.). But in this method, in order to crystallize the formed polylactic acid in the mold, until crystallization is completed, the formed body has to be held in the mold, so that the forming cycle becomes longer than during normal forming, and thus the manufacturing cost becomes high. Also, since it is necessary to heat the mold, a heating facility is also necessary.
Also, there is a method in which heat resistance is imparted by post-crystallizing a polylactic acid-family resin to a high degree by annealing it after forming. But in this method, in the process of post-crystallizing the formed body of a polylactic acid-family resin, the formed body may be deformed, so that problems arise in dimensional accuracy. Also, since a step for post-crystallizing is necessary, the manufacturing cost becomes high.
As another method of imparting heat resistance to a polylactic acid-family resin, there is known a method in which after making the storage elasticity in a predetermined range by pre-crystallizing a lactic acid-family polymer sheet by annealing, forming is carried out in a heated mold (see patent document 1). But in this method, too, in order to obtain a heat-resistant formed body, it is necessary to keep the temperature of the mold near the crystallizing temperature of the polylactic acid-family resin (80-130° C.), and finish crystallization of the polylactic acid-family resin in the mold, so that a facility for heating the mold is necessary. Also, in this method, the forming cycle becomes longer than during normal forming, so that the manufacturing cost becomes high.
Furhter, in the above-described method in which a polylactic acid-family resin is crystallized, the formed body obtained by the growth of spherutiles basically becomes opaque. Thus it is difficult to obtain a formed body having transparency.
On the other hand, as a technique regarding laminate sheets comprising a polylactic acid-family resin and a biodegradable aliphatic polyester other than a polylactic acid-family resin, there are known laminate films of aliphatic polyester having a number-average molecular weight of 10000 or over and polylactic acid-family resin (see patent document 2), and laminate films of polylactic acid-family resin and a biodegradable aliphatic polyester other than polylactic acid-family resin (see patent document 3).
(patent document 1) JP patent publication 8-73628
(patent document 2) JP patent publication 8-85194
(patent document 3) JP patent publication 10-6445